Crossed grid el display driver technique

ABSTRACT

Variable brightness, crossed grid, electroluminescent panel driving circuitry and logic means are described wherein the brightness of a selected cell is controlled by the length of driver circuit input pulse. The length of the input pulse is controlled by logic means.

United States Patent Munt Irwin Ellilbeih, N-J.

Mar. 23, 1970 Dec. 21, 1971 The United States of America Inventor ApplNo. Filed Patented Assignee CROSSED GRID EL DISPLAY DRIVER TECHNIQUE 3Claims, 5 Drawing Figs.

U.S. Cl 315/169, 340/335 Int. Cl 05b 37/00 Field of Search 315/169,

[.56] References Clted UNITED STATES PATENTS 3,343,l28 9/1967 Rogers3l5/l69X 3,069,596 i2/l962 Morgan 315/169 TV 3,054,929 9/1962 Livingston3 lS/i 69 Primary ExaminerRoy Lake Assistant Examiner-Lawrence J. DahlAttorneys-Richard S. Sciascia, John W. Pease and Harvey A.

David ABSTRACT: Variable brightness, crossed grid, electroluminescentpanel driving circuitry and logic means are described wherein thebrightness of a selected cell is controlled by the length of drivercircuit input pulse. The length of the input pulse is controlled bylogic means.

PATENTEU m2! 1921 3529553 sum 1 OF 3 je/ec e/ 6'(// IRWIN HUNT INVENTOR.

CROSSED GRID EL DISPLAY DRIVER TECHNIQUE BACKGROUND OF THE INVENTIONThis invention relates to electroluminescent displays of the typecomprising a matrix of display elements or cells which can beindividually accessed for excitation, and more particularly to thedriving of such a display in a manner that permits the brightness of theexcited cells to be varied in a controlled manner.

Electroluminescent displays utilizing a matrix or grid ofelectroluminescent cells or elements which may be individually accessedso as to generate pictures of other informational presentations havebeen known heretofore as exemplified by U.S. Pat. No. 3,054,929 of D. C.Livingston and U.S. Pat. No. 3,343,128, of R. J. Rogers. While it hasbeen known that the degree of brightness of an electroluminescent cellis dependent upon the exciting voltage thereacross, as discussed in U.S.Pat. No. 3,069,569 of D. W. Morgan, the prior art crossed grid, matrixtype of devices have been limited to voltage levels which produce eitheran on or an off condition of luminescence in each cell, with no degreesof brightness available in-between.

It has also been known to utilize the capacitive nature of the cells inresonant driver circuits which produce the necessary voltage levels toachieve luminescence in response to trigger or control pulses. Accessionof cells to be illuminated has been achieved by addition of voltagesapplied to opposite terminals of a selected cell. Again, there is noknown prior art wherein the resultant voltages can be selectively variedto produce varying degrees of brightness other than the full on or thefully dark condition.

BRIEF SUMMARY OF THE INVENTION With the foregoing in mind, it is aprincipal object of the invention to provide an improvedelectroluminescent display system utilizing driving methods andcircuitry which will achieve variable brightness in selected cells in acontrolled manner, thereby making it possible to generate realisticpictorial scenes on the display or otherwise increase the informationalcontent thereof.

It is another object of this invention to provide improved driving meansand techniques for a crossed grid electroluminescent display of the typeconcerned, wherein the variable brightness is controlled by utilizingexciting pulses for the resonant driver circuits, which pulses aredifi'erent lengths or durations corresponding to the degrees ofbrightness desired.

As another object the invention aims to provide novel digital logicmeans for generating a driver exciting pulse the length of which isvariable with respect to a reference length of pulse in response tobrightness level input signals.

BRIEF DESCRIPTION OF THE DRAWINGS The invention may be further said toreside in certain combinations, arrangements of parts, and methods aswill become apparent from the following description of a preferredembodiment when read in conjunction with the accompanying sheets ofdrawings, in which:

FIG. 1 is a diagrammatic illustration of electroluminescent drivercircuitry embodying the invention;

FIG. 2 is a graphic illustration of input pulses and output wavefonnsfor the circuitry of FIG. 1;

FIG. 3 is an illustration in block form of a display system utilizingthe invention;

FIG. 4 is a more detailed illustration in block form of the brightnesscontrol logic portion of the system of FIG. 3; and

FIG. 5 is a graphic illustration of the time relations of various pulsesin the logic portion illustrated in FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the form of the inventionillustrated in the drawings and described hereinafter, there is providedan electroluminescent display of the crossed grid type. That type ofdisplay is well known and comprises first and second spaced arrays ofparallel, separate conductors. The first array is illustrated ascomprising conductors Y Y,-Y,,, while the second array is illustrated ascomprising conductors X,, X, -X,, disposed in a direction normal to theconductors of the first array to form a plurality of crossover points. Adiscrete electroluminescent cell 11 is associated with each of thecrossover points such that application of a sufficiently highalternating current voltage across the conductors at any of thecrossover points will produce electroluminescence of the cell 11associated therewith. The degree of brightness of such illumination of aparticular cell is dependent upon the degree of voltage thereacross. Aplurality of the matrixes 10 can be assembled to provide a largerdisplay panel.

Associated with the array of Y conductors is a matrix driver means 12,and associated with the array of X conductors is a matrix driver means14.

In recognition of the capacitive nature of the individual cells of thedisplay, the driver circuits 12 and 14 which are about to be described,use resonating inductances in the form of stepup transformers togenerate the high voltages required. This configuration relies on anexciting pulse charging the capacitance of a selected cell, and alsoproducing a current flow in the associated inductance. The effect of thecurrent is to generate higher voltages than the stepped-up level of thepower supply. At the end of the charging pulse, the resonant circuitoscillates with the damping determined by the losses in the transformerand the capacitance. Practically, the oscillations disappear in abouttwo cycles.

The driver means 12 comprises a switching transistor 16 of the PNP-typehaving an emitter 18, base 20, and collector 22. The emitter isconnected at 24 to a suitable source of DC voltage, while the collector22 is connected through a rectifying diode 26 to one terminal of theprimary winding of a voltage step-up transformer 28. The base 20 of thetransistor 16 is connected via a resistor 30 to an input terminal 32.

The other terminal of the primary winding of the transfonner 28 isconnected to the collector 34 of a transistor 36 of the NPN-type. Thetransistor 36 includes an emitter 38 connected to ground, and a base 40connected to ground, and a base 40 connected through a resistor 42 to aninput terminal 44. The secondary winding of the voltage step-uptransformer 28 has one end connected to the conductor Y,, and the otherend connected to ground.

The collector 22 of the transistor 16 is further connected via conductor48 and a plurality of additional diodes 26'-26" to the primary windingsof additional transformers 28'-28" are connected to the conductors Y -YThe primary windings of the transformers 28 '-28 are connected to thecollectors 3434x of transistors 36'36", the emitters 38'-38" of whichare grounded and the bases of which are connected through resistors 42'-42" to input terminals 44'44".

Similarly, the driver means 14 for the array of X conductors comprises aswitching transistor 56 having its emitter 58 connected to a DC source60, its base 62 connected via a resistor 64 to an input terminal 66. Thetransistor 56 has its collector 68 connected through a diode 70 to oneterminal of the primary winding of a voltage step-up transformer 72. Theother terminal of the primary winding of the transformer 72 is connectedto the collector 74 of a transistor 76 of the NPN-type. The transistor76 includes an emitter 78 connected to ground, and a base 80 connectedthrough a resistor 82 to an input terminal 84. The secondary winding ofthe voltage step-up transformer 72 has one end connected to theconductor X, and the other end connected to ground.

The collector 68 of the transistor 56 is further connected via conductor88 and a plurality of additional diodes 7070" to the primary windings ofadditional transformers 72'-72", respectively. The secondaries of theseadditional transformers 72'72K are connected to the conductors X,-X,,.The primary windings of the transformers 72'-72" are also connected tothe collectors 74'74", respectively, of transistors 76'76, the emitters7878" of which are grounded, and the bases 80-80K of which are connectedthrough resistors 8282" to input terminals 84'-84' A selected cell isexcited by the sum of the voltages applied to the X and Y electrodes(conductors) defining the cell (in a coincident voltage fashion). Thevoltage applied to one of the coordinate conductors is treated as areference phase, while brightness is controlled by varying the width ofthe exciting pulse which operates the driver connected to the secondcoordinate conductor. Varying the'exciting pulse width varies thephasing of the oscillations generated and hence varies the sum of thevoltages between the X and Y conductors of a selected cell.

Assuming exciting pulses y; y,; .x; x to be applied to terminals 32, 44,66, and 84", respectively, the cell at the intersection of conductors Yand X, will be selected.

Referring to FIG. 2, it will be noted that the exciting pulses y; y,;and x are of the same time duration. This is treated as the referencetime. The pulses y; y produce an output on conductor Y as shown by thevoltage waveform 90.

The pulses x and x produce an output on conductor X, as shown by thevoltage waveform 92. It will be noted that because the pulse duration ofthe exciting pulse x is shorter than the reference period, the waveform92 is somewhat out of phase with waveform 90, and is characterized by asomewhat less peak voltage. The voltage across the selected cell isrepresented by the waveform 94.

It will be recognized that if the pulse x, were increased in length, thevoltage peaks of the waveform 92 would become more in time with those ofthe waveform 90. Moreover, the longer the duration of the pulse x thegreater will be the peaks of the waveform 92. The net result is anincrease in the voltage 94 across the selected cell and a concurrentincrease in brightness thereof. Conversely, shortening of the pulse xwill reduce the brightness level of the selected cell.

Referring now to FIG. 3, the driver exciting pulses may be derived fromX and Y line selection circuitry, generally indicated at 98 and 100,such as will, for example, provide a sequential sweep of the cells inthe display 10. The degree of brightness exhibited by a selected cell isadjusted by brightness (gray level) control logic 102 which serves toshorten or lengthen excitation pulses to the X terminals 84, 84, 84",etc., with respect to excitation pulses of reference length such asapplied to the terminals of the drive circuitry 12, in accordance withinformation, conveniently digital in form, received as shown by flowlines 104 from a source of gray level information 106.

The Y-line selection circuitry 98 and the X-line selection circuitry 100may also be digital in nature. The circuitry 98 and 100, and the source106 are not part of the invention per se but would be found in the typeof equipment with which the display is to be used, such as a flightsimulator for training pilots in landing field or aircraft carrierapproaches.

Referring to FIG. 4, an exemplary portion of the brightness controllogic 102 is illustrated. This logic comprises a flip-flop 110 whichreceives as one input a start pulse 112, as shown on base line (a) ofFIG. 5, via line 114 from the X-Iine selection circuitry. The startpulse 112 is coincident with the beginning of the x and y pulses onlines 116, 118 (FIG. 3) which are of a reference length 120 as shown bypulse y, on base line (b) of FIG. 5.

The flip-flop 110 has, as a second or reset input, the output on line124 of a register counter 126. The register counter 126 is conditionedby brightness code input, represented by lines 104, to provide an outputon line 124 when a predetermined number of clock pulses 128 are appliedthereto via line 130 and an AND-gate 132. The AND-gate 132 receives theclock pulses 128 via line 134 from a suitable clock pulse source 136.

These clock pulses are illustrated in FIG. 5 on a base line (0).

Upon the occurrence of a start pulse, the flip-flop is set to provide avoltage level change on line 138. This change constitutes the beginningof an X-line driver excitation pulse to the associated one of theterminals 84, 84, 84", etc. The

output of the fli -flop 110 is also fed via line 140 as a second inputto the A -gate which is enabled thereby to begin passing clock pulses128 to the register counter 126.

When the number of clock pulses 128 corresponds to the brightness codeinput via lines 104, the register 126 produces an output on line 124which resets the flip-flop 110 terminating the excitation pulse .x, andalso terminating the flow of clock pulses from AND-gate 132. It will beobserved from FIG. 5 that the pulse x may be varied by incrementscorresponding to the number of clock pulse periods within the referencepulse period. As described before, increasing or decreasing the pulselength of x, will increase or decrease the brightness of thecorresponding electroluminescent cell.

What is claimed is:

1. A variable brightness electroluminescent device comprismg:

first and second spaced electrodes with electroluminescent materialtherebetween and characterized by a predetermined capacitance,

first driver means comprising inductive means connected to said firstelectrode and operative in response to a first and second coincidentpulses having a predetermined reference length to provide an alternatingcurrent voltage to said first electrode of a frequency determined by thecapacitance of the cell and the inductance of said inductive means; and

second driver means comprising inductive means connected to said firstelectrode and operative in response to a third coincident pulse havingsaid reference length and to a fourth pulse which starts in coincidencewith said first, second, and third pulses but has a length which is lessthan said reference length to provide a second alternating currentvoltage to said second electrode, the peaks of which second alternatingcurrent voltage are out of phase with respect to said first alternatingcurrent voltage by an amount which varies with said length of saidfourth pulse.

2. A variable brightness electroluminescent device as defined in claim1, and further comprising:

logic means for generating said fourth pulse in response to a startsignal and so as to have a length corresponding to brightness levelinput signals to said logic means.

3. A variable brightness electroluminescent device as defined in claim2, and wherein said logic means comprises:

clock means for generating clock pulses at a predetermined frequency;AND-gate means connected to receive said clock pulses as a first inputthereto;

flip-flop means having first and second states and responsive toinitiate said fourth pulse as the second state output thereof;

said AND-gate means being responsive to said second state of saidflip-flop means to pass said clock pulses as an output of said AND-gatemeans;

register means connected to receive said clock pulses from said AND-gatemeans and conditioned by said brightness level input signals to providean output signal when a predetermined number of said clock pulses havebeen received; and

said flip-flop means being responsive to said output signal of saidregister means to return to said first state and terminate said fourthpulse.

1. A variable brightness electroluminescent device comprising: first andsecond spaced electrodes with electroluminescent material therebetweenand characterized by a predetermined capacitance; first driver meanscomprising inductive means connected to said first electrode andoperative in response to a first and second coincident pulses having apredetermined reference length to provide an alternating current voltageto said first electrode of a frequency determined by the capacitance ofthe cell and the inductance of said inductive means; and second drivermeans comprising inductive means connected to said first electrode andoperative in response to a third coincident pulse having said referencelength and to a fourth pulse which starts in coincidence with saidfirst, second, and third pulses but has a length which is less than saidreference length to provide a second alternating current voltage to saidsecond electrode, the peaks of which second alternating current voltageare out of phase with respect to said first alternating current voltageby an amount which varies with said length of said fourth pulse.
 2. Avariable brightness electroluminescent device as defined in claim 1, andfurther comprising: logic means for generating said fourth pulse inresponse to a start signal and so as to have a length corresponding tobrightness level input signals to said logic means.
 3. A variablebrightness electroluminescent device as defined in claim 2, and whereinsaid logic means comprises: clock means for generatinG clock pulses at apredetermined frequency; AND-gate means connected to receive said clockpulses as a first input thereto; flip-flop means having first and secondstates and responsive to initiate said fourth pulse as the second stateoutput thereof; said AND-gate means being responsive to said secondstate of said flip-flop means to pass said clock pulses as an output ofsaid AND-gate means; register means connected to receive said clockpulses from said AND-gate means and conditioned by said brightness levelinput signals to provide an output signal when a predetermined number ofsaid clock pulses have been received; and said flip-flop means beingresponsive to said output signal of said register means to return tosaid first state and terminate said fourth pulse.